This application is based upon and claims the benefits of priority from the prior Japanese Patent Application No. 2001-018934, filed on Jan. 26, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to generally to a semiconductor light emitting element.
2. Related Background Art
Semiconductor light emitting elements such as light emitting diodes (LEDs) and semiconductor laser diodes (LDs) have the features of small size, lightweight, low power consumption and high reliability. They are used in various applications as light sources for visible-light emitting, displaying and optical-communicating purposes. If the brightness of the light emitted increases, a range of applications as outdoor indicators and communications light sources will further increase. The materials of visible-light LEDs made practicable at present include various compound semiconductors such as AlGaAs, GaAlP, GaP and InGaAlP. They emit red, orange, yellow, green rays of light, etc., which are being supplied in low cost. Recently, blue light emitting diodes of gallium nitride compound materials have been developed, and three primary color LEDs, red LEDs, green LEDs and blue LEDs, have been already put to practical use.
Among these materials, the InGaAlP materials have direct transitional band compositions corresponding to red-green light wavelengths. Thus, they have been developed as LED materials that provide high internal quantum efficiency in red-green colors. FIG. 9 is a schematic cross-sectional view of an essential portion of a prior art InGaAlP semiconductor light emitting element, which includes an n-type InAlP clad layer 602, a InGaAlP active layer 603, a p-type InAlP clad layer 604, a p-type GaAlAs current spread layer 605 laminated in this order on an n-type GaAs substrate 601(a) p-side electrode 606 is formed on the current spread layer 605 and an n-side electrode 607 on the back of the substrate 601. In order to grow crystals of these materials, metal-organic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE) excellent in control of composition and film thickness is used. In growing the crystals, the GaAs substrate 601 used should match in lattice with the InGaAlP active layer 603.
In the conventional InGaAlP semiconductor light emitting element of FIG. 9, the GaAs substrate 601 is opaque to rays of light emitted from the active layer 603. Thus, the FIG. 9 element has the problem that the light extraction efficiency is not sufficiently high.
In order to increase the light extraction efficiency, a method has been proposed which includes removing the GaAs substrate 601 that absorbs rays of light emitted from the active layer 603, and providing a GaP substrate transparent to the rays of light from the active layer 603 instead. In this method, since the GaAs substrate 601 that absorbs rays of light from the active layer 603 is removed, the light extraction efficiency is improved considerably and provides the highest light output among the InGaAlP semiconductor light emitting elements provided so far. However, in order to make such composition, techniques for carefully handling thin eptaxial layers 602-605 after being removed the GaAs substrate 601(a)nd techniques for pasting the GaP substrate to a lamination of the eptaxial layers 602-605 were required. Therefore, the manufacturing process is complicated, thereby increasing the cost.
Another method has been proposed that includes forming a thick crystal layer on the active layer 603 to increase the light extraction efficiency. However, it is difficult to grow the thick crystal layer in the MOCVD or MBE, and hence this method is not so effective.
According to embodiments of the present invention, there is provided a semiconductor light emitting element comprising:
an active layer for emitting first light by current injection; and
a light absorbing and emitting section for absorbing one part of the first light and for emitting second light having a greater peak wavelength than the first light.
According to embodiments of the present invention, there is provided a semiconductor light emitting element comprising:
a GaAs substrate opaque to first light having a wavelength of xcex;
a light absorbing and emitting section formed on said GaAs substrate and matching in lattice with said GaAs substrate, said light absorbing and emitting section comprising an alternate lamination of first and second layers, each of said first layers having a refractive index of n1 and a thickness of xcex/4n1, each of said second layers having a refractive index of n2 (n2 less than n1) and a thickness of xcex/4n2, each of said second layers having a greater band gap than each said first layer;
a first conductive-type clad layer formed on said light absorbing and emitting section and made of a semiconductor matching in lattice with said GaAs substrate;
an active layer formed on said first conductive-type clad layer and made of a semiconductor matching in lattice with said GaAs substrate for emitting first light having a peak wavelength of xcex by current injection, said active layer having a band gap greater than each of said first layers of said light absorbing and emitting section and smaller than said first conductive-type clad layer;
said light absorbing and emitting section absorbing one part of the first light from said active layer and emitting second light having a greater peak wavelength than the first light, said light absorbing and emitting section also reflecting the remaining part of the first light from said active layer; and
a second conductive-type clad layer formed on said active layer and made of a semiconductor matching in lattice with said GaAs substrate, said second conductive-type clad layer having a greater band gap than said active layer.
According to embodiments of the present invention, there is provided a semiconductor light emitting element comprising:
a substrate made of any one of sapphire, GaN and SiC;
a light absorbing and emitting section formed on said substrate and comprising an alternate lamination of first and second layers, each of said first and second layers being made of a gallium nitride compound semiconductor, each of said first layers having a refractive index of n1 and a thickness of xcex/4n1, each of said second layers having a refractive index of n2 (n2 less than n1) and a thickness of xcex/4n2, each of said second layers having a greater band gap than each said first layer;
a first conductive-type clad layer formed on said light absorbing and emitting section and made of a gallium nitride compound semiconductor;
an active layer formed on said first conductive-type clad layer and made of a gallium nitride compound semiconductors, said active layer emitting first light having a peak wavelength of xcex by current injection, said active layer having a greater band gap than each of said first layers of said light absorbing and emitting section and smaller than said first conductive-type clad layer;
said light absorbing and emitting section absorbing one part of the first light from said active layer and emitting second light having a greater peak wavelength than the first light, said light absorbing and emitting section also reflecting the remaining part of the first light from said active layer; and
a second conductive-type clad layer formed on said active layer and made of a gallium nitride compound semiconductor, said second conductive-type clad layer having a greater band gap than said active layer.
According to embodiments of the present invention, there is provided a semiconductor light emitting element comprising:
a substrate made of any one of Si and GaAs, said substrate being opaque to first light having a wavelength of xcex;
a light absorbing and emitting section formed on said substrate and comprising an alternate lamination of first and second layers, each of said first and second layers being made of a gallium nitride compound semiconductor, each of said first layer having a refractive index of n1(a)nd a thickness of xcex/4n1, each of said second layers having a refractive index of n2 (n2 less than n1) and a thickness of xcex/4n2, each of said second layers having a greater band gap than each said first layer;
a first conductive-type clad layer formed on said light absorbing and emitting section and made of a gallium nitride compound semiconductor;
an active layer formed on said first conductive-type clad layer and made of a gallium nitride compound semiconductors, said active layer emitting first light having a peak wavelength of xcex by current injection, said active layer having a greater band gap than each of said first layers of said light absorbing and emitting section and smaller than said first conductive-type clad layer;
said light absorbing and emitting section absorbing one part of the first light from said active layer and emitting second light having a greater peak wavelength than the first light, said light absorbing and emitting section also reflecting the remaining part of the first light from said active layer; and
a second conductive-type clad layer formed on said active layer and made of a gallium nitride compound semiconductor, said second conductive-type clad layer having a greater band gap than said active layer.